Various devices have been conventionally employed to measure the level of a fluid or the interface levels between two mediums (such as oil/water). Generally, these devices consist of a sensor within a container, and means for sending data from the sensor to a location where it would be detected and converted into a usable format representative of the level of fluid within the container. One common device is a time domain reflectometry device having a waveguide positioned in the tank (or a side chamber) and a signal generator and signal receiver. The device generates an electromagnetic signal which propagates down the waveguide. Upon reaching a fluid interface of fluids having differing dielectric constants, a portion of the signal is reflected from the interface and portion is transmitted through the interface and continues down the waveguide. The reflected signals are received, and from these reflected signals, the reflected times can be used to calculate fluid levels and interface relationships, all as well known in the art.
In containers having three fluids, such as air/oil and oil/water, it is desirable to track each interface to be able to calculate the volume of the fluids stored in the container. Each interface will create a reflected signal, provided the dielectric constants of each fluid are sufficiently different to create a contrast at the interface strong enough to produce a reasonable strength reflected signal. Even with a strong reflection at the first interface, a portion of the signal is transmitted through the interface, and continues traveling through the tank to the second interface. Again, if the dielectric constants of the second and third layers are sufficiently different, a reflected signal will be created at the interface. It is desired that the dielectric constants of the upper products be less than that of the lower products, and it is preferred that the difference in dielectric constants be greater than 10. Preferred ranges are as follows: generally, if a gas fluid such as air is the first layer, the dielectric constant will be about 1.0; the second fluid dielectric constant preferred range is 1.3 and 5.0; while the third fluid layer should have a dielectric constant of about greater than or equal to 15. Such ranges will generally produce readily detectable reflections at the air/liquid 1 and liquid 1/liquid 2 interfaces and the air/liquid 1 reflected signal will be of smaller amplitude than that from the liquid 1/liquid 2 interface.
In such a system, detection of a returned signal requires detection of the reflected signal and clocking the time (either absolute time (time from a fixed starting point, such as emission time) or differential time (time between reflected signals)). Due to noise present in the system, detection of a reflected signal generally requires detection of a signal having an amplitude or signal strength that exceeds a threshold value. When two reflections are expected, two different threshold values are preferred to account for the differences in the signal strengths of the reflections. As indicated above, the reflection from the liquid 1/liquid 2 interface is anticipated to be of greater strength than that from the air/liquid 1 interface, and hence the threshold 1 (air/liquid 1) is set at less that threshold 2 (liquid 1/liquid 2). In general, the air or gas/liquid interface references the “total level” in the tank.
Even when the stored products have suitable dielectric constants, the liquid/liquid interface may not be crisp or well defined due to interaction between the two liquids. For instance, in a container having an oil layer floating on a water layer, the oil/water interface may be diffuse due to mixing or the presence of an emulsion layer. Such a diffuse interface layer creates a weak return signal. For instance, the top of the emulsion layer may be detected as liquid/liquid interface, or conversely, an emulsion layer may prevent the creation of a sufficient reflected signal. A signal may not be reflected at the emulsion interface as the dielectric change through the emulsion layer is insufficient to create a strong reflected signal because the dielectric change is spread out instead of being localized. It is desirable to be able to detect the potential presence of the emulsion layer as an indicator that the level reading may be compromised.